This invention relates to a radio frequency receiving system and, more particularly, to such a system wherein a phase modulated pseudonoise sequence signal is received.
In certain inter-vehicular communication systems, phase modulated pseudo-random noise coded pulses are used for both initial acquisition and data. An acquisition phase is used to acquire time synchronization and frequency correction of the received signal, after which data can be decoded by conventional means. The initial frequency uncertainty may be due to a combination of Doppler frequency shift due to vehicular motion and/or to local oscillator offset or drift. In such a system, it is typical that the vehicle wishing to receive data stepwise rotates its directional receiving antenna and examines any incoming signal to determine whether an appropriate signal is received. During the acquisition phase, the transmitter sends a phase modulated pseudonoise sequence signal, which is a specially coded signal. The receiver must therefore determine whether that coded signal has been received and, if so, whether the frequency of the received signal is such that it can be decoded without undue error.
It is therefore an object of the present invention to provide a low cost hardware implementation of a system for determining the acquisition of a phase modulated pseudonoise sequence signal when the time of arrival and frequency offset are unknown.
It is a further object of the present invention to provide such an arrangement which inherently compensates for frequency offset of the received signal so that errors due to frequency offset are reduced.